(a) Field of the Invention
The present invention relates to an improved bright zinc electroplating bath and method which includes the use of (i) condensation products of aromatic aldehydes with amides and (ii) condensation products of aromatic aldehydes with urea or substituted ureas.
(b) Prior Art
Zinc plating from acid solutions has been known in the industry for many years, with applications generally limited to high speed plating, such as strip steel plating. In this application where plating is done at extremely high current densities, only a very narrow bright range is obtainable unless organic brightening agents are employed. The bath commonly contains sulfate ions or is a mixture of sulfate and chloride ions. Deposits from these baths are dull in the mid-to-low current density range and, therefore, are not suitable for decorative applications where low current densities are encountered or complicated by shaped parts which must be bright after plating. Attempts to overcome the deficiencies of sulfate baths with organic brighteners have met little commercial success, since poor throwing power is inherent in the electrolyte itself.
Decorative zinc plating became possible with the introduction of cyanide baths, but such baths had to be operated at a pH greater than 7.0 to prevent the generation of hydrogen cyanide gas. Bright deposits were produced consistently with little attention required for maintenance of the bath as long as the sodium cyanide concentration was maintained above 20-25 grams/liter. Organic addition agents were developed which, in conjunction with the brightening effect of the cyanide ion itself, produced extremely bright deposits from these alkaline solutions.
When consideration of the environment became an integral part of plating operations, the earliest attempts were to run the cyanide baths at lower concentrations. This can be done conveniently to a certain lower limit at which point the brighteners are no longer effective. New additives have been introduced since that time which may employ cyanide concentrations in the bath as low as one gram/liter; however, these baths sacrifice the ease of operation which for so long a period of time was their most significant commercial advantage. Cleaning of parts prior to plating became a critical factor in the successful operation of low cyanide baths. Today, virtually all plating shops must chemically destruct cyanide-containing effluents before they can be discharged. Chemical destruction is an expensive additional cost to a business and its elimination would be desirable.
Recently, non-cyanide alkaline zinc baths have been developed but with only minor success, mainly due to the high susceptibility of the bath to contamination by metals and organics which makes control difficult and the cleaning of parts prior to plating extremely critical.
All alkaline baths have the added disadvantage of low cathode efficiency (in the order of 30-35% in the case of cyanide baths) and great difficulty in plating cast iron parts. Poor cathode efficiencies can not be overcome by any means and the plating of cast iron parts from alkaline baths requires a special "activation" step.
Neutral zinc baths were developed, based on complexing agents to keep the zinc ions in solution at a pH 6.0 to 8.0. These complexing agents make waste treatment, by dilution or by increasing the pH, impossible because the zinc ions are still held in solution. These complexing agents are not specific for zinc but will "tie up" other metal ions in the effluent as well, such as copper or nickel from other operations.
Mildly acidic ammonium chloride baths were introduced which depended on the ammonium ions to complex the zinc. These baths suffer from the same disadvantage in waste treatment, ie., the ammonium ions will complex other metals in the effluent, complicating the treatment process.
A recent innovation in acid zinc plating is non-ammoniated baths which use potassium or sodium ions as substitutes for the ammonium ions. These baths have no complexing agents and waste treatment may easily and economically be done by pH adjustment. The chief limiting factor associated with these processes is the stability of the organic additives used to produce the bright deposits. Common materials employed are aromatic aldehydes and ketones, as described in U.S. Pat. Nos. 3,594,291; 3,694,330; 3,729,394; 3,891,520; and 4,070,256. In each process described, mixtures of surface-active agents and carboxylic acids are employed with aldehydes and ketones. Surface-active agents mentioned most frequently are non-ionic compounds, made by ethoxylating or propoxylating various starting materials, such as nonyl phenol, 2-naphthol, ethylene diamine, ethanol, ethylene glycol, etc. The aldehydes and ketones are slightly soluble oils in aqueous solutions, such as plating baths; therefore, a solvent, miscible with water, is used to dissolve them. Breakdown products from these aldehydes and ketones are even less soluble in aqueous solutions and will float on the surface of the bath as an oil, adhering to racks of parts, which ultimately contaminates the entire system and results in pitted and blotchy deposits. Skimming and filtration are required to eliminate these oils from the bath and it can not be used for production while this cleaning operation is in progress.
A second limitation associated with these processes is that they will produce deposits that are highly stressed and have poor ductility. This is critical in those instances where thicker than normal deposits are required.
A third limitation is the high cost of operation of these processes where expensive solvents are required to keep the brighteners dissolved in the bath. In many cases, a true solution is not achieved, as can be seen by cloudy plating solutions.
A fourth limitation is the narrow temperature range in which these processes must be operated (20.degree.-27.degree. C.). In areas where ambient temperatures approach 32.degree. C., it is very difficult to maintain an operating temperature of 27.degree. C. in the bath, and expensive cooling equipment must be used.
A fifth limitation associated with these processes is the relatively narrow operating range of cathode current density. It is desirable to plate at the highest possible current density so that a shorter plating time can be used to give the desired deposit thickness. Burning occurs at high current densities when the limiting current density of a bath is exceeded. This is accentuated when chemical-conserving baths are used where the zinc ion concentration is as low as 30-35 grams/liter. Ideally, the weakest solution possible to plate a metal satisfactorily is desirable in order to lessen the amount of metal ions lost to the waste treatment system. The limiting current density is not only affected by the concentration of zinc ions but has also been shown to be a function of the types or organic additives employed.
Accordingly, it is the principal object of this invention to provide a zinc acid plating bath which contains organic brighteners which do not suffer from the problems associated with the hereinbefore discussed prior art brightener systems.
It is also an object of this invention to provide a stable non-ammoniated acid zinc bath.
It is further an object of the invention to provide a zinc plating bath containing additives which do not produce insoluble oils as products of electrolysis with prolonged use, thereby eliminating the associated problems and purification procedures.
It is another object of the invention to provide electrodeposits with good brightness, ductility, and stress properties.
It is another object of the invention to provide a zinc plating bath which contains additives which are effective brighteners at all practical current densities and maintain a clear, colorless, zinc plating solution indicative of the fact that the compounds are soluble.
It is another object of the invention to provide a zinc plating bath capable of operating at higher temperatures, such as 27.degree. to 32.degree. C.
It is still another object of the invention to provide a zinc plating bath capable of plating at higher current (faster deposition rate) without producing burned deposits in the higher current density areas.
It is another object of the invention to provide a zinc plating bath which is economical to operate in that a low concentration of zinc ions may be used in making up the bath, which also reduces waste treatment costs.
Still another object of the invention is to provide an improved method for obtaining bright zinc electrodeposits.
Other objects of the invention will become apparent to those skilled in the art from a reading of the following specifications and claims.